[Python-ideas] Proposal: Use mypy syntax for function annotations

[Andrew Barnert]

On Aug 13, 2014, at 21:06, Jukka Lehtosalo <jlehtosalo at gmail.com> wrote:

> You could use AnyStr to make the example work with bytes as well:
> 
>   def word_count(input: Iterable[AnyStr]) -> Dict[AnyStr, int]:
>       result = {}  #type: Dict[AnyStr, int]
>       for line in input:
>           for word in line.split():
>               result[word] = result.get(word, 0) + 1
>       return result

Defining a function as taking an Iterable[AnyStr] and returning a Dict[AnyStr, int], without any way to declare that the two AnyStr are the same type, is exactly what I meant by losing critical information. I pass in something that I know is a text file, and I get out something that are either strings or bytes and I don't know which; what am I going to do with that? If you can't propagate types, static typing doesn't help.

My point is that the depth you seem to be reaching for is not actually a sweet spot for power vs. simplicity. You could make it simpler by just dropping generics in favor of a handful of special-purpose modifiers (optional, iterable-of), or you could make it more useful by having real parametric types, or at least the equivalent of C++ template templates or Swift arbitrary constraints. What makes Java-style simple generics with subclass constraints the right level of complexity?